One of the biggest problems for manufacturers of mufflers is controlling the sound level in an exhaust system while at the same time keeping the exhaust flow at a sufficiently high level to produce good power output. Part of the problem is that the relationship between engine hard-parts technology and exhaust technology is complex and not fully understood or quantified. A comprehensive analysis of exhaust flow requires a consideration of many factors, such as exhaust air flow, pressure, heat, sound, frequencies, sound energy and exhaust pulses.
To understand better what happens in an exhaust system, consider what happens to one exhaust pulse from the cylinder where it begins, until it exits into the atmosphere. As an exhaust stroke is being completed, the exhaust valve opens and an exhaust pulse exits into the exhaust system. This pulse, for example, can be like a tennis ball traveling down the exhaust pipe. At the moment the pulse exits the cylinder, it can be traveling at almost 1,000 feet per second with sound energy accompanying it. Directly behind the pulse, a low pressure area is created.
The further away from the cylinder the pulse travels, the more heat and speed it loses. Anything that creates back pressure will slow its progress even more. As the pulse reaches the end of the exhaust pipe and exits into the atmosphere, the low pressure area behind the pulse is suddenly replaced by atmospheric pressure. Timing the pulses to exit at regular intervals is important for good performance as it reduces back pressure by keeping the low pressure area in the pipe helping to pull the next pulse through the exhaust system.
Controlling exhaust pulse timing while attempting to control sound levels or sound energy within acceptable limits can be difficult, and doing so without an accompanying loss of power and air flow through the engine is one of the more difficult problems faced by muffler manufacturers. My previous U.S. Pat. No. 4,574,914, which is the subject of Reexamination Certificate No. 1599, discloses an earlier attempt of mine to develop a muffler that is effective in attenuating sound and which can even reduce back pressure when used on certain high performance engines. The muffler of my '914 patent is based, in part, on the principle of first dividing incoming exhaust gases and then reconverging them back together prior to releasing the exhaust gases from the muffler.
My more recent U.S. Pat. No. 5,444,197 improves upon the concept and design of my earlier '914 patent. My '197 patent incorporates an intermediate reflector partition between the divided exhaust gases and the converging exhaust gases. This intermediate partition directs portions of the sound components in the exhaust gases away from the muffler outlet opening.
While the mufflers of my '914 patent and my '197 patent are highly effective and in wide-spread use in both racing and street vehicles, it is always highly desirable to further reduce muffler back pressure and at the same time further attenuate sound components entrained in exhaust gases. In addition, for certain applications, my prior muffler designs are too quite. A certain amount of low RPM engine "rumble" on the outside of the vehicle without resonating in the interior of the vehicle can be desirable. For this reason, it is desirable to be able to tune the sound frequencies to a particularly pleasing frequency profile.
It is an object of the present invention to provide a method and apparatus for tuning the sound frequency profile of an engine exhaust system to desirable levels.
It is another object of the present invention to provide a method and apparatus for decreasing the overall sound levels produced by an engine exhaust system.
It is another object of the present invention to provide a method and apparatus for controlling low pressure regions in an engine exhaust system.
It is another object of the present invention to achieve one or more of the foregoing objects with a muffler that is inexpensive to manufacture and install, yet which is durable enough in construction to withstand the harsh environment of an engine exhaust system.
These and other objects of the invention will become apparent from the following Disclosure of the Invention and Best Mode section and the accompanying drawings.